We are studying the mechanisms whereby proteins assemble into biological membranes. This study is employing the major coat protein of the bacteriophage M13. This coat protein is an integral protein of the host cell cytoplasmic membrane at each stage of the virus infection. Coat protein is initially made as a soluble cytoplasmic precursor which binds to the cytoplasmic membrane and, in a reaction which requires the electrical potential across the membrane bilayer, inserts into the membrane and is processed to its mature form.